" 2. Arachnomorphae.
Order 4. Palpigradi (= Microthelyphonidae).
Order 5. Solifugae (= Mycetophorae).
Order 6. Pseudoscorpiones (= Chelonethi).
Sub-order a. Panctenodactyli.
" b. Hemirtenodactyli.
Order 7. Podogona (= Ricinulel).
Order 8. Opiliones.
Sub-order a. Laniatores.
" b. Palpatores.
" c. Anepignathi.
Order 9. Rhynchostomi (= Acari).
Sub-order a. Notostigmata.
" b. Cryptostigmata.
" c. Metastigmata.
" d. Prostigmata.
" e. Astigmata.
" f. Vermiformia.
" g. Tetrapoda.

CLASS. ARACHNIDA.--Euarthropoda having two prosthomeres (somites which
have passed from a post-oral to a prae-oral position), the appendages
of the first represented by eyes, of the second by solitary rami which
are rarely antenniform, more usually chelate. A tendency is exhibited
to the formation of a metasomatic as well as a prosomatic carapace by
fusion of the tergal surfaces of the somites. Intermediate somites
forming a mesosoma occur, but tend to fuse superficially with the
metasomatic carapace or to become co-ordinated with the somites of the
metasoma, whether fused or distinct to form one region, the
opisthosoma (abdomen of authors). In the most highly developed forms
the two anterior divisions (tagmata) of the body, prosoma and
mesosoma, each exhibit six pairs of limbs, pediform and plate-like
respectively, whilst the metasoma consists of six limbless somites and
a post-anal spine. The genital apertures are placed in the first
somite following the prosoma, excepting where a praegenital somite,
usually suppressed, is retained. Little is known of the form of the
appendages in the lowest archaic Arachnida, but the tendency of those
of the prosomatic somites has been (as in the Crustacea) to pass from
a generalized bi-ramose or multi-ramose form to that of uni-ramose
antennae, chelae and walking legs.

The Arachnida are divisible into two grades of structure--according to
the fixity or non-fixity of the number of somites building up the
body:--

_Grade A_ (_of the Arachnida_). _ANOMOMERISTICA._--Extinct archaic
Arachnida, in which (as in the Entomostracous Crustacea) the number of
well-developed somites may be more or less th